B Klasse Diesel Spongy Brake Pedal

I had a similar issue with my B Klasse. The strange pedal feel, especially. It turned out to be a combination of things. Have you checked the condition of your brake lines? Sometimes a small leak can introduce air into the brake system, leading to that spongy feeling. It's worth a look before you start tearing things apart.

I have not inspected the brake lines directly, as that would require more sophisticated tools, but I did not notice any visible leaks around the calipers or master cylinder. Could a minor leak truly manifest in such a pronounced change in pedal feel? I am reluctant to start replacing parts without more certainty.

A minor leak can definitely cause that. Air in the lines is a pain. But if you're not seeing anything obvious, and you suspect undervoltage, it might be best to have the system properly diagnosed at a workshop. They can check the ABS module and other components that rely on consistent voltage. It's better than chasing ghosts and potentially making things worse.

I appreciate the advice. I shall schedule an appointment with a trusted mechanic to perform a comprehensive evaluation of the brake system, including the ABS module and the brake lines. It turned out it was indeed the ABS module. The undervoltage caused the ABS to malfunction. Repair cost me 240€